Semiconductor devices are becoming smaller and more dense with the evolution of new technology. Therefore, there is a pressure for this industry to constantly make improvements and innovation in this technology. The semiconductor manufacturers are therefore constantly being challenged to improve the quality of their products by identifying and eliminating defects and to make improvements where necessary. Whereas significant improvements are being made to eliminate systematic defects by reducing process variability. Process improvements alone are not sufficient to eliminate all the random defects which effect both yield and reliability. Historically, inspection techniques have been employed to improve product failure rates to acceptable levels by culling out many of these random defects.
Similarly, in the fabrication of ceramic substrates, such as, multilayer ceramic (MLC) substrates, the formation of vias is critical to the electrical performance of the final substrate. Vias can be formed by a number of methods. The most popular method of forming vias in ceramic green sheets is by punching or drilling holes through the individual green sheets and filling these holes with a metallurgical paste that is electrically conductive.
These individual green sheets are then laminated by methods well known in the art and the green sheet laminates are then processed through a furnace. After sintering of the laminated green sheets and the subsequent formation of a ceramic substrate, the vias containing the metal form an electrical path between the various layers on the substrate.
U.S. Pat. No. 3,892,614 (Levy), discloses a electrostatic laminating apparatus and method in which two or more sheets to be pinned together are passed in close proximity or in contact with each other through an electric field and the resulting electrostatic charge pins these two sheets together.
U.S. Pat. No. 4,457,972 (Griffith), teaches adhesion of layers using high energy bombardment. Basically, films are firmly bonded to insulator substrates by irradiating the interface with high energy ions.
U.S. Pat. No. 5,167,997 (Chamberlain), discloses the formation of protected conductive foil assemblage using static electrical forces. A plastic film is removably joined with an electrically conductive foil. The absence of air between the film and the foil is produced by oppositely statically electrically charging the film and the foil such that they are forced together sufficiently to squeeze the air out from therebetween.
IBM Technical Disclosure Bulletin, "Backing Material For MLC Screening", Vol. 24, No. 10, page 5119 (March 1982), teaches that backing material is typically employed in the MLC (Multi-Layer Ceramic) screening processes. It also teaches that the backing material prevents paste at the via holes from contacting the screening nest. And, that the backing material should also exhibit porosity or perforations for vacuum hold down of the green sheet and it should have non-porous edges for vacuum handling. Additionally, it discloses that the backing material utilized for this prior art screening processes should exhibit low static charge susceptibility, is smooth and planar, and is compatible with the screening paste used, and also exhibits dimensional stability.
In most of the prior art processes pits and/or depressions are commonly formed during the screening process. The formation of pits or depressions in the green sheet screening process is basically associated with volumetric changes during the drying of the paste. During the drying cycle the solvents are removed and the paste volume decreases. Additionally, pits are also formed if the interface between the paste and the backing film is somehow degraded. However, if the paste adheres to the backing film and it easily separates upon drying then pitting is minimized. This fixed interface forces the volumetric shrinkage to occur at the opposite surface of the green sheet which is the actual bottom surface of the top layer of the completed substrate.
The present invention relates to a new method and apparatus for the fabrication of green sheets for MLC applications which contain a flush, pit-free via surface. This is accomplished by applying a temporary electrostatic charge to the ceramic green sheet and a polymeric or insulating backing film. The electrostatic charge results in an electrostatic adhesion of the two layers or material. This adhesive force prevents the movement of the green sheet relative to the backing film, thus maintaining a good interface between the two films/sheets. After the screening process has been performed the electrostatic charge is removed and subsequently the backing layer is peeled off, leaving behind a green ceramic sheet with pit-free vias.